1. Field of the Invention
The present invention relates to an electronic pressure gage intended primarily for use by people performing refrigeration service. The gage has a replaceable data memory. The data memory has stored therein saturated temperatures corresponding to pressures for a particular volatile fluid or refrigerant. The invention further relates to such gages that include a temperature measuring instrumentality. The invention further relates to such gages having a display and means for displaying any of pressure, temperature, saturated temperature corresponding to observed pressure or the numerical difference between such saturated temperature and the observed temperature. The invention further relates to such gages which further include memory means and means for storing and subsequently displaying the highest and lowest values of any displayable parameter.
2. Prior Art
Instruments for measuring superheat at the suction outlet of refrigeration evaporators are well known. One such instrument includes two temperature sensors, one to be placed at the refrigerant inlet of the evaporator and the second to be placed at the refrigerant or suction outlet. If the temperature at the suction outlet is higher than the temperature at the refrigerant inlet, the difference is considered to be the superheat. This system produces faulty results when the evaporator is designed with a high pressure drop or where the refrigerant is neither a pure fluid nor an azeotropic mixture. Other instruments embody pressure transducers and temperature transducers plus integrally stored switch selected data relating observed pressure with corresponding saturated temperature. One version of this type instrument can display both the observed data and the stored data and perform calculations involving both. One commercially available product has a memory within the gage casing having switch selected data for several volatile fluids or refrigerants.
Mechanical Pressure Gages:
Most refrigeration service people still employ mechanical gages with analog output. Such gages have Bourdon tubes actuating a rotary pointer which moves over a circular scale calibrated in pounds per square inch (psi) for pressures above atmospheric pressure and in inches of Mercury vacuum for pressures less than atmospheric pressure. A pressure five psi above atmospheric pressure would indicate 5 on the gage. A pressure five psi less than atmospheric would indicate 10 inches Mercury column vacuum or approximately 2 inches Mercury column for each psi. A perfect vacuum would read 30 inches of Mercury vacuum.
Mechanical refrigeration service gages, in addition to pressure markings, generally include scales from which saturated temperatures for common refrigerants such as CFC-12 (dichlorodifluoromethane) and HCFC-22 (monochlorodifluoromethane) can be read.
Azeotropes and Non-Azeotropes (Zeotropes):
In the past, every refrigerant either was a pure chemical or a constant boiling mixture of pure chemicals. Examples of pure chemicals employed as refrigerants are dichlorodifluormethane (CFC-12) and propane (HC-290). A mixture of chemicals which exhibits the same boiling and condensing characteristics as a pure chemical is called an azeotrope. An example of an azeotropic refrigerant is a mixture of HCFC-22 and CFC-115 (chloropentafluoroethane). A 50--50 mixture of these two refrigerants is known by the shorthand designation of R-502. Both pure chemicals and azeotropes have only a single saturated temperature corresponding to any pressure.
Many New Refrigerants, Some are Zeotropes:
In the past the period between the introduction of new refrigerants usually corresponded to the lifetime of the equipment. However, with the discovery by Roland and Molina of the potential devastating effect of chlorofluorocarbon refrigerants (CFC's) on the stratospheric ozone layer and with the effectuation of the Montreal Protocol in which most of the producing nations of the world agreed to stop production of CFC refrigerants, there came a flood of new refrigerants. Each month new refrigerants are offered to the trade, each new refrigerant having a new composition and requiring new characteristics relating saturated temperature corresponding to observed pressure. Further, because of the desire of refrigerant manufacturers to provide new, non-ozone damaging, refrigerants whose characteristics match those of the soon-to-be-banned CFC refrigerants, many of the new refrigerants are mixtures of fluids which are not azeotropes and therefore do not act like pure fluids in that they do not have a single saturated temperature corresponding to a single pressure. These mixtures are called non-azeotropes or zeotropes.
Zeotropic Mixtures:
Zeotropic mixtures do not have a single saturation temperature corresponding to a single pressure. Instead a sample of such a zeotropic fluids boils and condenses away over a range of temperatures, called "the glide".
Zeotropic Mixtures: Bubble Point, Mid-point, Dew Point:
When a zeotropic mixture is held at a fixed pressure and warmed, the first bubbles appear at the "bubble point temperature". The last vestige of liquid boils away at a higher temperature called the "dew point temperature". The average between the bubble point temperature and the dew point temperature is called the mid point temperature.
Saturation Temperature for Performance:
In refrigeration design, testing and service, knowledge of an accurate value of saturated temperature corresponding to an observed or predicted pressure is very important. Values of saturated temperature are almost always employed instead of actual measured temperatures, in tests for determining the performance of refrigeration evaporators, where transitions from liquid to vapor, and of condensers, where transitions from vapor to liquid, are occurring. Also, the values of saturated temperature are always employed where a measure of the superheated condition the condition of a vapor or the subcooled condition of a liquid is sought.
Saturation Temperature and Superheat:
At the refrigerant vapor outlet of most evaporators it is expected that only vapor will be present. The number of degrees that the vapor is warmer than the saturation temperature corresponding to the actual pressure of the vapor is called the vapor superheat. The superheat is a measure of how much of the evaporator is effective in the cooling process. A high superheat suggests that much of the evaporator is not being used for evaporation, a highly effective mode of heat transfer, but instead is being employed for superheating the refrigerant vapor, a heat transfer mode which is much less effective than the evaporation mode.
Saturation Temperature and Subcooling:
At the refrigerant liquid outlet of condensers, where it is expected that only liquid refrigerant will be present. The number of degrees that the liquid temperature is cooler than the saturation temperature corresponding to the liquid pressure is called the liquid subcooling. A high degree of subcooling suggests that much of the condenser is not being used for condensing vapor, an effective heat transfer mode, but instead is being employed for subcooling, a much less effective mode of heat transfer.
Dew Point, Mid Point and Bubble Point Temperatures; Uses:
When employing zeotropic mixtures as refrigerants, the dew point temperature corresponding to a given pressure is employed to determine superheat; the bubble point temperature corresponding to a given pressure is employed to determine subcooling; and the mid point temperature is employed to determine performance.
Effect of Altitude on Gage Readings:
When the service person read her analog mechanical gage, she assumes that the pressure she reads is the pressure inside the tube or vessel to which her gage is attached. This is a valid assumption when she and the gage she is using are at sea level. In that case the reference pressure seen by the outside of the Bourdon tube is about 14.7 psi and the pressures indicated by the needle are indicated with reference to that reference pressure.
However, when the service person is in Denver, the atmospheric pressure is about 2.5 psi less than atmospheric. When the service-persons gage reads 0 psi, the actual pressure in the system she is measuring is not 14.7 psia but 12.2 psi. When performing service on an air-conditioner employing HCFC-22 as refrigerant, this difference will result only in about a 3.5 F error in estimating the saturation temperature of the refrigerant. However, if the service person is attempting to analyze a problem with a centrifugal compressor employing CFC-11 (trichlorofluoromethane) the 2.5 psi difference will result in a 14 F mistake in estimating saturation temperature, a potentially disastrous difference.
Altitude Correction:
Refrigeration service-people generally have in their possession printed cards called pressure-temperature charts. these charts have printed thereon tables, setting forth for a number of refrigerants, a wide range of gage pressures and corresponding saturated temperatures. These charts are correct only when the gage used to measure the pressures for entry into the chart is at sea level. At altitudes higher or lower than sea level the service person must correct her gage reading by the difference between the atmospheric pressure at her location and atmospheric pressure at sea level.
Need for New Gage, Objectives of Present Invention:
In view of the difficulties of accurate pressure measurement in refrigeration systems, the proliferation of many new refrigerants, the need to be able to use a gage accurately at altitudes, the need to conform to stringent governmental codes, some of which having criminal consequences, the need to cope with zeotropic refrigerants having more than one important temperature corresponding to each pressure, and the need to enable lesser skilled service personnel to observe and reach accurate conclusions about the systems they are observing, the present invention has the following objectives:
To provide an electronic pressure gage with an addressable data memory for displaying saturated temperature as well as pressure.
To provide such a gage having temperature sensing means and means for displaying the difference between measured temperature and saturated temperature.
To provide such a gage where the data memory is an external replaceable element, whereby new elements containing characteristics of new refrigerants can be secured.
To provide such a gage where maximum and minimum values of a displayable variable can be stored and displayed.
To provide a gage where either superheat or subcooling can be measured and displayed and identified.
To provide such a gage capable of interfacing with and providing accurate superheat and subcooling measurements with zeotropic as well as pure and azeotropic refrigerants.
To provide such a gage having a removable temperature sensor where the gage senses the presence of the sensor and displays superheat or subcooling when the temperature sensor is present and saturated temperature and pressure only when the sensor is absent.
To provide such a gage having a pressure sensor which responds to absolute pressure, further including zeroing means whereby the absolute value of the ambient atmospheric pressure is read before pressure connection to a source is made and the value of the ambient pressure is stored.
To provide such a gage where the displayed pressure is gage pressure, that is the pressure with respect to the ambient atmospheric pressure, while the saturated temperature, displayed and employed for superheat calculations, is based on the absolute pressure.
To provide such a gage where the external memory containing the refrigerant characteristics is applicable to zeotropic refrigerants and where the external memory for such refrigerants includes both the dew point temperature and the bubble point temperature for each pressure.
To provide such a gage where the external memory containing the refrigerant characteristics is applicable to zeotropic refrigerants and where the external memory for such refrigerants includes either the dew point temperature or the bubble point temperature for each pressure.
To provide such a gage which displays the midpoint temperature corresponding to a measured pressure when the temperature sensor is disconnected.
To provide such a gage which displays and employs the dew point temperature when the observed temperature is higher than the dew point temperature and employs and displays the bubble point temperature when the observed temperature is lower than the bubble point temperature.
Further advantages and objectives will become apparent as the design, construction and operation of the invention is more fully disclosed herein.